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This photograph shows a fountain of liquid magma during the eruption of Stromboli volcano in Italy.
Click on image for full size Image from Dr. J. Alean, Eglisau, Switzerland For more information, images and video about Stromboli volcano see Stromboli On-Line.

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Extrusive Igneous Rocks

Extrusive igneous rocks, or volcanics, are formed when magma makes its way to Earth's surface, erupts or flows above the surface as lava, and then cools forming rock. The lava that erupts onto the Earth's surface can come from different levels of the Earth's upper mantle, between 50 km and 150 km below the surface. It is under high pressure because of the weight of the rocks above and the pressure will allow it to come to the surface if it can find a route up.

When lava erupts onto the Earth's surface, it cools quickly because the temperature above the surface is much lower than below. If the lava cools very quickly, in less than a day or two, there is no time for elements to bond forming minerals. Instead, elements are frozen in place within volcanic glass. Often, lava cools over a few days to weeks and minerals have enough time to form but not time to grow into large crystals. These rocks with tiny crystals are called aphanitic or fine-grained.

Basalt is the most common type of extrusive igneous rock and the most common rock type at the Earth's surface.

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